[pgpool-hackers: 3395] Re: Failover consensus on even number of nodes

[Tatsuo Ishii]

>>> Hi Ishii-San,
>>> 
>>> On Sat, Aug 17, 2019 at 1:00 PM Tatsuo Ishii <ishii at sraoss.co.jp> wrote:
>>> 
>>>> > Hi Ishii-San
>>>> >
>>>> >
>>>> > On Thu, Aug 15, 2019 at 11:42 AM Tatsuo Ishii <ishii at sraoss.co.jp>
>>>> wrote:
>>>> >
>>>> >> Hi Usama,
>>>> >>
>>>> >> When number of Pgpool-II nodes is even, it seems consensus based
>>>> >> failover occurs if n/2 Pgpool-II agrees on the failure. For example,
>>>> >> if there are 4 nodes of Pgpool-II, 2 nodes agree on the failure,
>>>> >> failover occurs. Is there any reason behind this? I am asking because
>>>> >> it could easily lead to split brain, because 2 nodes could agree on
>>>> >> the failover while other 2 nodes disagree. Actually other HA software,
>>>> >> for example etcd, requires n/2+1 vote to gain consensus.
>>>> >>
>>>> >>
>>>> >>
>>>> https://github.com/etcd-io/etcd/blob/master/Documentation/faq.md#what-is-failure-tolerance
>>>> >>
>>>> >> With n/2+1 vote requirements, there's no possibility of split brain.
>>>> >>
>>>> >>
>>>> > Yes, your observation is spot on. The original motivation to consider the
>>>> > exact n/2 votes for consensus rather (n/2 +1)
>>>> > was to ensure the working of 2 node Pgpool-II clusters.
>>>> > My understanding was that most of the users use 2 Pgpool-II nodes in
>>>> their
>>>> > setup, so I wanted
>>>> > to make sure that in the case when one of the Pgpool-II nodes goes down (
>>>> > In 2 node) cluster the consensus
>>>> > should still be possible.
>>>> > But your point is also valid that makes the system prone to split-brain.
>>>> So
>>>> > what are your suggestions on that?
>>>> > I think we can introduce a new configuration parameter to enable/disable
>>>> > n/2 node consensus.
>>>>
>>>> If my understanding is correct, current behavior for 2 node Pgpool-II
>>>> clusters there's no difference whether failover_when_quorum_exists is
>>>> on or off. That means for 2 node Pgpool-II clusters even if we change
>>>> n/2 node consensus to n/2+1 consensus, 2 node users could keep the
>>>> existing behavior by turning off failover_when_quorum_exists. If this
>>>> is correct, we don't need to introduce the new switch for 4.1, just
>>>> change n/2 node consensus to n/2+1 consensus. What do you think?
>>>>
>>> 
>>> Yes, that's true, turning off the failover_when_quorum_exists will
>>> effectively give us the
>>> same behaviour for 2 nodes cluster.
>>> 
>>> 
>>>> The only concern is 4 node Pgpool-II clusters. I doubt there's 4 node
>>>> users in the field though.
>>>>
>>> 
>>> Yes, you are right there wouldn't be many users who would deploy 4 nodes
>>> cluster. But somehow we need
>>> to keep the behaviour and configurations consistent for all possible
>>> scenarios.
>>> 
>>> Also, the decision of considering either n/2 or (n/2 +1) as a valid
>>> consensus for voting is not only limited to
>>> the backend node failover. Pgpool-II also considers the valid consensus
>>> with n/2 votes when deciding the
>>> watchdog master. And currently, the behaviour of watchdog master elections
>>> and backend node failover consensus
>>> building is consistent. So If we want to revisit this we might need to
>>> consider the behaviour in both cases.
>> 
>> Ok, it seems creating new parameter for switching n/2 or n/2+1 could
>> be safer, I agree. Usama, would like to implement this for 4.1?
> 
> Attached is a proof of concept patch. GUC and doc change are not
> included. With the patch, 2 watchdog node cluster will go into "quorum
> absent" state if one the nodes goes down.

Attached is ready for review patch. GUC and English manual included.

Best regards,
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese:http://www.sraoss.co.jp
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